1. Field of the Invention
This invention relates to radar systems, and more specifically to radar systems that are used in applications that require low cost, low power consumption, low probability of detection, jam resistance, or where another radar system is present.
2. Background Information
The first applications for radar focused on the detection of airborne objects at large range. Today, this powerful tool has been applied to many other useful applications. Examples include burglar alarms, systems used to determine the speed of moving vehicles, and modules that are used to open automatic doors.
Many of these applications arose as the complexity and cost of radar technology was reduced. The use of new solid state devices for the generation and detection of rf and microwave radiation, as well as the proliferation of microprocessors, digital signal processors, and application specific integrated circuits have contributed to the availability of radar systems that are simpler and less expensive. As the price of radar technology continues to fall, new applications will come into play.
Many of these applications will benefit from low power consumption, especially if the application requires the use of battery power. One example is an automatic faucet control for a sink. The control module is required to turn on the water when a person has placed his hand under the faucet. Ideally, the module will be battery powered, and have a battery life in excess of one year. For applications such as these, low power consumption is critical.
In certain applications, the radar must operate without being detected. Additionally, the radar must be resistant to both unintentional and intentional interference. Improvements in these areas will result in radar systems with greater utility.
There are a number of emerging applications that require operation of two or more radar sensors in the same general location. One example of this is for vehicular collision avoidance systems in automobiles and trucks. Each radar system must be able to operate in an environment where many other radar transmitters are present. A second example is in support of sensor networks. Sensor networks may involve the use of two or more collocated radar systems, and so each radar sensor must operate properly in the presence of the other.
A number of radar applications require high range resolution for detection of intended targets. Range resolution is defined as the minimum resolveable distance between two point scatterers separated in range. Resolution of several feet may be adequate for some radars intended to simply detect airborne targets, while resolutions of one foot or less may be desired for other applications such as detection of targets in ground clutter or for use in generating distributed range return signals which can be used for discriminating between different targets.
The conceptually simplest method to achieve high range resolution is to transmit a very short radio-frequency pulse. Individual scatterers which are separated in range by distance greater than the pulse width can be resolved upon reception by the radar of their reflections of this short pulse. Historically, these very short pulses have been difficult to generate with sufficient power for use in practical radar systems.
Over the past several years, development of new technologies have enabled the transmission and reception of very short radio-frequency pulses and their use in practical radars. These methods have come to be known as Ultra-Wideband (UWB) for the extremely wide bandwidths necessarily present in the spectra of these pulses.
Standard UWB radar designs exploit their high bandwidth in order to transmit a narrow impulse type signal. This narrow pulse allows high range resolution returns from targets while maintaining low transmitted power. The following are some examples of radar designs which transmit and receive a very short UWB pulse having a pulse width that defines the system bandwidth: U.S. Pat. No. 3,772,697, entitled BASE BAND PULSE OBJECT SENSOR SYSTEM by Ross; U.S. Pat. No. 4,651,152, entitled LARGE RELATIVE BANDWIDTH RADAR by Harmuth; U.S. Pat. No. 5,095,312, entitled IMPULSE TRANSMITTER AND QUANTUM DETECTION RADAR SYSTEM by Jehle, et. al.; U.S. Pat. Nos. 5,345,471 and 5,523,760, entitled ULTRA-WIDEBAND RECEIVER by McEwen; U.S. Pat. No. 5,361,070, entitled ULTRA-WIDEBAND RADAR MOTION SENSOR by McEwen; U.S. Pat. No. 5,543,799, entitled SWEPT RANGE GATE RADAR SYSTEM FOR DETECTION OF NEARBY OBJECTS by Heger; U.S. Pat. Nos. 6,177,903 and 6,400,307, entitled SYSTEM AND METHOD FOR INTRUSION DETECTION USING A TIME DOMAIN RADAR ARRAY by Fullerton et. al.; and U.S. Pat. No. 6,208,248, entitled QUICK RESPONSE PERIMETER INTRUSION DETECTION SENSOR by Ross. The techniques provided in each of these UWB approaches involve sending a single pulse that utilizes the entire system bandwidth, and the resulting resolution is based upon the system bandwidth.
Additional methods construct and transmit very short pulses using very wide bandwidths through addition and transmission of combinations of carrier waves at discrete frequencies, such as described in U.S. Pat. No. 5,239,309, entitled ULTRA WIDEBAND RADAR EMPLOYING SYNTHESIZED SHORT PULSES by Tang, et. al. and “Ultra-Wideband Radar Using Fourier Synthesized Waveforms” by Gill et. al., IEEE Transactions on Electromagnetic Compatibility, vol. 39, no. 2, May 1997, pgs. 124–131. This results in the shaping of a single UWB pulse which utilizes the entire system bandwidth, upon which the resulting radar resolution is based.
Conventional radars which transmit low resolution narrowband pulses can also achieve high effective range resolution through the use of pulse compression techniques. Pulse compression techniques require the transmission of a signal or set of signals (typically, several hundred radar pulses) comprising a wide frequency band, each of the signals is narrowband and has a duration that is longer than an expected range delay extent for an intended target. Postprocessing of the multiple received low resolution return signals from these signals is used to synthesize range return delay profiles whose resolution is on the order of the inverse of the transmitted/received bandwidth.
One particular type of pulse compression radar is known as a stepped frequency radar. The term stepped frequency refers to the fact that multiple frequencies are transmitted in a stepped (sequential) order, with a fixed frequency separation between successive pulses. In conventional stepped frequency radars, in order to produce a valid range versus return waveform, each fixed frequency transmitted pulse is longer than the expected range delay extent of the target or range information which is to be acquired. Examples of such stepped frequency pulse compression radar systems in which the pulses are longer than the expected range delay extent for the intended target are described in: U.S. Pat. No. 4,450,444, entitled STEPPED FREQUENCY RADAR TARGET IMAGING by Wehner; U.S. Pat. No. 5,499,029, entitled WIDE BAND STEPPED FREQUENCY GROUND PENETRATING RADAR by Bashforth, et. al.; U.S. Pat. No. 5,592,170, entitled RADAR SYSTEM AND METHOD FOR DETECTING AND DISCRIMINATING TARGETS FROM A SAFE DISTANCE by Price, et. al.; and “A new millimeter-wave step-frequency radar sensor for distance measurement”, Joongsuk Park; Cam Nguyen, Microwave and Wireless Components Letters, IEEE, vol. 12 Issue: 6, Jun. 2002, pp. 221–222.
A concept similar to this is called hopped-frequency in which the individual narrowband frequency pulses are sent in a non-sequential or randomized order. This can be accomplished in the same manner as step frequency radars with simple modifications to the transmitter and receiver. A number of methods use continuous frequency modulation, the most common being linear frequency modulation (LFM), to achieve resolution equivalent to transmitting and receiving a pulse which uses the entire bandwidth. One example of hopped frequency approach using LFM is described in D. R Wehner, “High Resolution Radar”, 2nd Edition, Artech House, Boston, 1995, pgs. 149–161. A similar example describes continuous amplitude modulation, which is described in A. W. Rihaczek, “Principles of High Resolution Radar”, McGraw Hill, 1969, pgs. 226–255. Similarly, all pulses in these approaches have pulse durations that are longer than the expected range delay extent for the intended targets.
There are a number of methods which combine the various methods of step frequency, linear frequency and/or amplitude modulation to achieve resolution equivalent to transmitting and receiving a pulse which uses the entire bandwidth. Some examples are: U.S. Pat. No. 3,945,012, entitled WIDE BAND PULSED ENERGY SYSTEM by Cooper; U.S. Pat. No. 3,987,285, entitled DIGITAL MATCHED FILTERING USING A STEP TRANSFORM PROCESS by Perry; U.S. Pat. No. 4,309,703, entitled SEGMENTED CHIRP WAVEFORM IMPLEMENTED RADAR SYSTEM by Blahut; and U.S. Pat. Nos. 5,867,117 and 6,225,941, entitled SWEPT-STEP RADAR SYSTEM AND DETECTION METHOD USING SAME by Gogineni et. al. Again, all of these approaches must use narrowband pulses having a pulse width which is longer than the target range extent in order to produce a useful range versus scattered radar return waveform.
The pulse compression process which is applied to the stepped frequency and/or hopped frequency radar return signals is essentially the same. At each frequency, a coherent demodulator is employed to compute an in-phase (I) and quadrature-phase (Q) pair of values from each transmitted/received low resolution pulse. We denote these by Ik and Qk, where k=0, 1, 2, . . . , N−1 represents the kth frequency transmitted and received.
Conventional pulse compression processing for stepped and hopped frequency radars is based on a model of the reflected radar returns which assumes that the narrowband returned signal from an extended target is comprised of multiple reflections of the transmitted narrowband sinusoidal pulse which add coherently with different phases. The I and Q value sampled from the return at each frequency can be interpreted as a sample of the Fourier domain representation of the range profile. Thus, the basic premise behind pulse compression for stepped and hopped frequency radars is that the return signal represents a sampled frequency response, from which a range profile is constructed. Accordingly, the duration of the pulses is required to be longer than the expected range delay extent for the intended targets.
An Inverse Discrete Fourier Transform             H      l        =                  ∑                  k          =          0                          N          -          1                    ⁢                          ⁢                        (                                    I              k                        +                          jQ              k                                )                ⁢                  ⅇ                                    j              ⁡                              (                                  2                  ⁢                                      π                    /                    N                                                  )                                      ⁢            lk                                ,typically implemented using a Fast Fourier Transform (FFT), is applied to the frequency domain values to compute the range delay response signal at finitely many range values (see for example, U.S. Pat. No. 4,450,444). The values H1 are referred to as the target's complex range profile. Often, it is simply the magnitude of these values which is used to determine if a target is present in a detection application, for example. Standard techniques such as zero padding the frequency domain samples for improving the sampling frequency in the synthesized time signal and windowing of the frequency domain samples to reduce sidelobes of the synthesized impulse response are often employed, such as described by Harris, Fredric J., “On the Use of Windows for Harmonic Analysis with the Discrete Fourier Transform”, Proceedings of IEEE, January 1978, pgs. 51–83.
Another approach using conventional narrowband radar pulses having a duration longer than an expected range delay extent of the intended target is described in U.S. Pat. No. 3,299,427, entitled RADAR SYSTEM by Kondo. Kondo describes radar in which the transmitted signal consists of a train of narrowband pulses of different frequency whose transmission and return at each frequency is delayed so as to produce an alignment of returned signals equivalent to what would occur if all pulses were transmitted simultaneously with no delays in reception.
In contrast to conventional radar systems sending narrowband pulses, it cannot be assumed that a transmitted UWB pulse is longer than the range delay extent of a target of interest. In fact, UWB pulses are typically very much shorter than the range delay extents of almost all targets of interest; thus, pulse compression methods as employed in conventional systems using narrowband pulses, such as a stepped radar approach, are not used with radar systems using very short pulsewidth UWB signals since it would violate the basic assumption behind pulse compression.